Multiply.
Explanation: $12 \times 158$ and $1.2\times 15.8$ multiply the same digits in the same order. So, the product of both problems will also have the same digits in the same order. Let's multiply $12 \times 158$. Then we can estimate to place the decimal point in the product of $1.2 \times 15.8$. $\begin{aligned} {1}58&\\ \underline{ \times {1}2}&\\ {16}& {2} \times {8\text{ ones}}\\ {100}& {2} \times {5\text{ tens}}\\ {200}& {2} \times {1\text{ hundred}}\\ {80}& {10} \times {8\text{ ones}}\\ {500}& {10} \times {5\text{ tens}}\\ \underline{+{1{,}000}}& {10} \times {1\text{ hundred}}\\ \end{aligned}$ $\begin{aligned} 158&\\ \underline{ \times 12}&\\ 16}& {2} \times {8\text{ ones}}\\ 100}& {2} \times {5\text{ tens}}\\ 200}& {2} \times {1\text{ hundred}}\\ 80}& {10} \times {8\text{ ones}}\\ 500}& {10} \times {5\text{ tens}}\\ \underline{+1{,}000}}& {10} \times {1\text{ hundred}}\\ 1{,}896 \end{aligned}$ $12 \times 158 = 1{,}896$ Let's estimate to place the decimal in $1.2\times 15.8$. $\begin{aligned} 1.2 \times 15.8 &\approx 1\times 16\\\\ &\approx 16 \end{aligned}$ Where can we place the decimal in $1{,}896$ to get a product close to $16$ ? $18.96 = 1.2 \times 15.8$